Okay, here's a first attempt at a fully worked out Goat Demon. You have to look pretty closely to see the SRD Ghoul that is under it!
I will admit it owes at least as much to watching Monkey episodes on TV to the original monster's description. Particularly in the Illusory Disguise and the weapon-wielding.
Goat Demon
Medium Undead (Extraplanar, Shapechanger)
Hit Dice: 2d12
Initiative: +2
Speed: 30 ft. (6 squares) [40 ft. in goat form]
Armor Class: 17 (+2 Dex, +5 natural), touch 12, flat-footed 15
Base Attack/Grapple: +1/+3
Attack: Horns +3 melee (1d8+3 plus paralysis) or halberd +3 melee (1d10+3/×3) or slam +3 melee (1d3+2)
Full Attack: Horns +3 melee (1d8+3 plus paralysis) and 2 slams +1 melee (1d3+1); or halberd +3 melee (1d10+3/×3) and horns +1 melee (1d8+1 plus paralysis)
Space/Reach: 5 ft./5 ft.
Special Attacks: Paralysis, inexorable gnawing
Special Qualities: Alternate form, darkvision 60 ft., illusory disguise, undead traits, +2 turn resistance
Saves: Fort +0, Ref +2, Will +5
Abilities: Str 14, Dex 15, Con –, Int 13, Wis 14, Cha 12
Skills: Balance +6, Climb +5, Hide +6, Jump +5, Move Silently +6, Spot +7
Feats: Multiattack
Environment: Any land or The Abyss
Organization: Solitary or herd (2-24)
Challenge Rating: 1
Treasure: Standard
Alignment: Always Chaotic Evil
Advancement: By character class
Level Adjustment: —
Goat demons are a minor form of undead that originate from an unknown layer of the Abyss. They delight in the destruction of mankind. Some goat demons hope to work their way up the hierarchy of the damned by the corruption and damnation of living sapients, especially clerics, but most are crude monsters who achieve little beyond the slaughter of the living and desecration of the dead. Goat Demons frequently chew their way into graves and tombs to defile and devour the contents.
Goat demons speak Abyssal and Common.
Combat
A goat demon usually assumes some innocent looking guise - such as an old woman, a stray goat or a large dog - and tries to work its way close to a lone victim, then launch a surprise attack with its paralysing horns. If it succeeds it uses Inexorable Gnawing on its helpless victim. A herd of goat demons use similar ambush tactics, often masquerading as a goatherd and their flock or a band of travellers.
Goat demons are not brave creatures, they will switch to quadruped form and flee if wounded or outmatched.
Alternate Form (Su): A goat demon's natural form is that of a hideous decomposing goat, but it prefers to attack in its alternate form, a creature with the body of a hairy ghoul and the head of a goat (stats as above). A goat demon can switch from one form to the other with a swift action. When in its true goat form the goat demon's Speed increases to 40 ft., but it loses the ability to wield weapons (or any other activity requiring hands).
If slain, a goat demon reverts to its goat form and then dissolves into a pile of bones and putrescence.
Illusory Disguise (Su): The goat demon can take the appearance of any Small or Medium-sized creature or object that is shaped like a humanoid or a four-legged animal. The goat demon can take on the form of inanimate objects of the same shape (i.e. statues) as well living, dead or undead creatures. This is an illusion (glamour) effect that includes visual, olfactory (smell), auditory (sound) and tactile (touch) components. A goat demon using this power to create a disguise gets a +10 bonus on its Disguise check. A creature that interacts with the Illusory Disguise gets a DC12 Will save to recognize it as an illusion.
Inexorable Gnawing (Ex): A goat demon can chew its way through almost anything with its powerful jaws and teeth. As a standard action, the goat demon can automatically do 2d6+3 damage to any immobile object, including victims of its Paralysis attack and opponents the goat demon has grappled and pinned. Inexorable Gnawing ignores any Hardness and Damage Reduction that has a value less than 15.
Paralysis (Ex): Those hit by a goat demon's horns attack must succeed on a DC 12 Fortitude save or be paralyzed for 1d4+1 rounds. The save DC is Charisma-based.